The human eye in its simplest terms functions to provide vision by transmitting light through a clear outer portion called the cornea, and focusing the image by way of a lens onto a retina. The quality of the focused image depends on many factors including the size and shape of the eye, and the transparency of the cornea and lens. Age and/or disease often cause the lens to become less transparent. Thus, vision deteriorates because of the diminished light which can be transmitted to the retina. This deficiency in the lens of the eye is medically known as a cataract. An accepted treatment for this condition is surgical removal of the lens and replacement of the lens function by an IOL.
The majority of ophthalmic lenses, including IOLs, currently used are of a monofocal design, (i.e., having a fixed focal length). The focal length of the implanted IOL generally is chosen to optimize vision at a single distance from the patient. Thus, most patients receiving an IOL still require glasses for clear distance and near vision.
Multifocal ophthalmic lens designs generally fall into one of two categories, refractive lenses and diffractive lenses. Diffractive lenses use nearly periodic microscopic structures on the lens to diffract light into several directions simultaneously. This is similar to a diffraction grating and the multiple diffraction orders focus the light into various images corresponding to different focal lengths of the lens. Diffractive multifocal contact lenses and IOLs are more fully discussed in U.S. Pat. Nos. 4,162,122, 4,210,391, 4,338,005, 4,340,283, 4,995,714, 4,995,715, 4,881,804, 4,881,805, 5,017,000, 5,054,905, 5,056,908, 5,120,120, 5,121,979, 5,121,980, 5,144,483, 5,117,306 (Cohen), U.S. Pat. Nos. 5,076,684, 5,116,111 (Simpson, et al.), U.S. Pat. No. 5,129,718 (Futhey, et al.) and U.S. Pat. Nos. 4,637,697, 4,641,934 and 4,655,565 (Freeman), the entire contents of which are incorporated herein by reference.
While a diffractive IOL may have a number of focal lengths, generally, IOLs with only two focal lengths (far and near) are the most common. As with any simultaneous vision multifocal lens, a defocused image (or images) is superimposed on the focused component because of the second lens power, but the defocused image is rarely observed by the user, who concentrates on the detail of interest. Under certain circumstances (for example, at night), the pupil diameter of the user can expand to 5 millimeters (mm) or more, and a discrete distant light source (e.g., automobile headlights or street lights) can appear to be surrounded by a “halo” or “rings”. A significant component of the halo is caused by the light that is directed to the near image which becomes defocused at the retina. The visibility of the halo is affected by the diameter of the lens region directing light to the near image, the proportion of total energy directed to the near image, and the overall imaging aberrations of the eye.